Fatty acids of specific types are of considerable interest both for the maintenance of healthy tissue and in the treatment of various diseases. Some fatty acids are of interest in themselves, some because they give rise to particular metabolites such as prostaglandins or other oxygenated derivatives, and some for both reasons. Among them are the essential fatty acids (EFAs) not made by the body and therefore constituting essential nutrients. Among the EFAs of particular interest for both reasons are gamma-linolenic acid (GLA), dihomogammalinolenic acid (DGLA) and eicosapentaenoic acid (EPA). DGLA is an important component of cell membranes and is also the precursor of prostaglandin E.sub.1 (PGE.sub.1); PGE.sub.1 has many desirable effects being an antithrombotic, anti-inflammatory, vasodilator, immunomodulating and cholesterol lowering agent. GLA is an immediate precursor of DGLA and is rapidly converted to DGLA within the body. EPA is also a component of cell membranes and is a precursor of PGE.sub.3 which has actions similar to those of PGE.sub.1. A further fatty acid which is of particular interest as a component of cell membranes is docosahexaenoic acid (DHA).
The conversation pathways of the main series of EFAs in the body are as follows:
TABLE 1 ______________________________________ n-6 n-3 ______________________________________ 18:2 delta-9,12 18:3 delta-9,12,15 (linoleic acid) (alpha-linolenic acid) .vertline. delta-6 desaturase .dwnarw. 18:3 delta-6,9,12 18:4 delta-6,9,12,15 (gamma-linolenic acid) (stearidonic acid) .vertline. elongation .dwnarw. 20:3 delta-8,11,14 20:4 delta-8,11,14,17 (dihomo-gamma-linolenic acid) .vertline. delta-5 desaturase .dwnarw. 20:4 delta-5,8,11,14 20:5 delta-5,8,11,14,17 (arachidonic acid) (`eicosapentaenoic acid`) .vertline. elongation .dwnarw. 22:4 delta-7,10,13,16 22:5 delta-7,10,13,16,19 (adrenic acid) .vertline. delta-5 desaturase .dwnarw. 22:5 delta-4,7,10,13,16 22:6 delta-4,7,10,13,16,19 (`docosahexaenoic acid`) ______________________________________
The above pathways are not normally reversible nor, in man, are n-3 and n-6 series acids inter-convertible.
The acids, which in nature are of the all-cis configuration, are systematically named as derivatives of the corresponding octadecanoic, eicosanoic or docosanoic acids, eg. delta-9,12-octadecadienoic acid or delta-4,7,10,13,16,19-docosahexaenoic acid, but numerical designations such as, correspondingly, 18:2 n-6 or 22:6 n-3 are convenient. Initials, for example, EPA for the 20:5 n-3 acid (eicosapentaenoic acid) or DHA for the 22:6 n-3 acid(docosahexaenoic acid), are also used but do not serve when n-3 and n-6 acids of the same chain length and degree of unsaturation exist as for example with the 22:5 acids. Trivial names in more or less common use in the n-6 series are as shown. Of the n-3 series only 18:3 n-3 has a commonly used trivial name, alpha-linolenic acid, though the name stearidonic acid is coming into use for the 18:4 n-3 acid and the names eicosapentaenoic acid and docosahexanenoic acid as such are also used. The alpha isomer of linolenic acid was characterised earlier than gamma-linolenic acid and reference in the literature simply to linolenic acid, especially in the earlier literature, is to the alpha-acid.